G'day La Nina: Christmas to be cooler, wetter this year

JUST in time for Christmas, the weather has gifted Australia an early present with the declaration the country is now officially in the grip of La Nina.

But whether the La Nina climate driver will be a much cherished Christmas present, or the equivalent of yet another pair of socks, will depend very much on where you live.

While it's a bit early to forecast exactly what the weather will be like on Christmas Day, La Nina is leading the meteorological boffins to firm up their thoughts on December's weather - as well as for the rest of summer.

La Nina has been declared, now sea temperatures have cooled to -0.8C below average in the equatorial Pacific. Picture: Bureau of Meteorology.

"We're going to experience La Nina this Christmas. In December, the odds are over 60 per cent for a wetter, colder December in southern Queensland, NSW, Victoria, Tasmania and the populated areas of South Australia," Dr Andrew Watkins, the Bureau of Meteorology's (BoM) Manager of Long Range Forecasts told news.com.au.

It could be cloudier too, with less blue skies. However, the most southerly capitals also have an increased chance of longer, warmer spells this Christmas. But these are unlikely to be, "short, sharp extreme temperature" spikes, Dr Watkins said.

Partly cloudy. Medium (50%) chance of showers and the chance of a thunderstorm during this afternoon and evening. Severe thunderstorms are possible.

Light winds becoming northwest to northeasterly 15 to 25 km/h in the middle of the day then becoming northwesterly 15 to 20 km/h in the late evening.

Daytime maximum temperatures in the low to mid 30s.

Mid-North Coast

Partly cloudy. Medium (60%) chance of showers and the chance of a thunderstorm during this afternoon and evening.

Severe thunderstorms are possible. Winds north to northwesterly 15 to 25 km/h tending northwest to southwesterly in the early afternoon then tending southeast to southwesterly in the late afternoon.

Daytime maximum temperatures 29 to 34.

Reader poll

Are you looking forward to a cool, wet Christmas?

This poll ended on 30 December 2017.

Current Results

YES

69%

NO

30%

This is not a scientific poll. The results reflect only the opinions of those who chose to participate.

The Australian Bureau's La Nina confirmation came a bit later than the organisation's US counterparts which require equatorial sea surface temperature to sink to 0.5C below average. The bar is higher, and cooler, for the BoM which says the same seas must be -0.8C below.

But the outcome for Australia is same. During a La Nina, which has counter effects to El Nino, while equatorial seas get cooler, the waters north of Australia warm up. That helps to produce rising air, clouds and rainfall over northern and eastern parts.

"Typically, the La Nina impacts we see is that summers are cooler and a bit wetter," Dr Watkins said.

"But this year's a bit different as the La Nina is looking weak and it's starting much later which is quiet unusual."

Looking forward to this on Christmas Day in Bondi? Picture: Dylan Robinson

The late La Nina, as well as colder than expected seas around Northern Australia, means the effects are unlikely to be at the extreme end of the scale and could even weaken further, bringing conditions closer to average.

There’s a chance it could be more like this in Bondi over Christmas.

Dr Watkins said several effects of La Nina were already being felt. The heatwaves and rains in Victoria and Tasmania had La Nina written all over them.

But there were subtler signs too, which could mean Bondi may not be the best location for a Christmas Day dip.

"Summer swimming has come early around Tasmania and Victoria with waters 3C warmer than usual but around Sydney, seas are 3C cooler so it's a bit chilly. Port Phillip Bay is actually warmer than Sydney Harbour," he said.

This isn't unheard of during a La Nina with winds coming off the Tasman Sea cooling down Sydney and Brisbane. Melbourne, Adelaide and Hobart miss out on these while also being affected by slow moving warm weather systems lolloping lazily across the continent.

The Bureau will release its official Christmas Day forecast on 18 December, but here's the information we have so far on how La Nina could affect the capitals over the festive season.

Sea surface temperatures around Victoria and Tasmania are warmer than around Sydney. Picture: Bureau of Meteorology.

There wasn't a strong swing to either exceptionally dry or wet conditions in WA but Dr Watkins said there was a 70-75 per cent chance Perth would see a drier than average December.

"La Nina and El Nino have less of an impact (in WA); it's a bit more influenced by what's happening in the eastern Indian Ocean. At the moment, the ocean is slightly cooler than normal which is pretty abnormal and the opposite of what we expect to happen."

Where's the monsoon? That's the question in the Top End. Normally at this time of year there would be signs of the monsoon forming but it currently remains north of the equator. Yet, it could still make an appearance in time to dampen Christmas lunch.

Monthly rainfall totals can be quite variable during the wet season, for example in Darwin the average December rainfall is 254mm but the highest recorded at 665mm in 1974 and the lowest just 19mm in 1991. The weak La Nina is expected to have a minimal effect on rainfall.